Capacitor voltage transformer are used for line voltmeters, synchroscopes, protective relays, tariff meter, etc.
The capacitor voltage transformer is more economical than an electromagnetic voltage transformer when the nominal system voltage increases above 66 kV.
The carrier current equipment can be connected via the capacitor of the Capacitor Voltage Transformers. Thereby there is no need of separate coupling capacitors.
Capacitor Voltage Transformers also serve as coupling capacitors for coupling high frequency power line carrier signals to the transmission line.
CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.
A voltage transformer VT is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal, for measurement or to operate a protective relay.
The performance of capacitor voltage transformer (CVT or CCVT) is inferior to that of electromagnetic voltage transformer. Its performance is affected by the supply frequency, switching transients, magnitude of connected burden, etc.
The performance of capacitor voltage transformer (CVT or CCVT) is inferior to that of electromagnetic voltage transformer. Its performance is affected by the supply frequency, switching transients, magnitude of connected burden, etc.
The capacitor voltage transformer is more economical than an electromagnetic voltage transformer when the nominal system voltage increases above 66 kV.
The carrier current equipment can be connected via the capacitor of the Capacitor Voltage Transformers. Thereby there is no need of separate coupling capacitors.
Capacitor Voltage Transformers also serve as coupling capacitors for coupling high frequency power line carrier signals to the transmission line.
CVTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.
Capacitor type VT is used for voltages 66 kV and above. At such voltages cost of electromagnetic type VT’s tends to be too high.
Working of CVT
The capacitors connected in series act like potential dividers provided the current taken by the burden is negligible compared with the current passing through the series connected capacitors.
However, the burden current becomes relatively larger and ratio error and also phase error is introduced. Compensation is carried out by ‘tuning’.
The reactor connected in series with the burden is adjusted to such a value that at supply frequency it resonates with the sum of two capacitors. This eliminates the error.
The construction of capacitor type VT depends on the form of capacitor voltage divider. Generally, h.v. capacitors are enclosed in a porcelain housing. A large metal sheet box at the base encloses the tuning coil intermediate transformer.
However, the burden current becomes relatively larger and ratio error and also phase error is introduced. Compensation is carried out by ‘tuning’.
The reactor connected in series with the burden is adjusted to such a value that at supply frequency it resonates with the sum of two capacitors. This eliminates the error.
The construction of capacitor type VT depends on the form of capacitor voltage divider. Generally, h.v. capacitors are enclosed in a porcelain housing. A large metal sheet box at the base encloses the tuning coil intermediate transformer.
In an electrical power substation, Capacitor Voltage Transformer in combination with Wave Trap is placed at the sending and receiving ends of the substation. At the receiving end they are found just after lightening arrester and before line isolator.
Construction of CVT (CCVT)
Capacitor Voltage Transformers consist of two primary assemblies,
- the high voltage capacitor sections and
- the base box, housing the electromagnetic components.
The General arrangement of a capacitor voltage transformer is given below,
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| General Arrangement of Capacitor Voltage Transformer |
Series connected capacitor elements, housed in porcelain shells, each hermetically (in airtight manner) sealed, are referred to as capacitor sections. The dielectric of the capacitor elements is made up of high quality polypropylene film/paper and impregnated with highly processed synthetic fluid.
Each capacitor section is equipped with a stainless steel below which will allow the synthetic fluid to expand and contract with changes in ambient operating temperature while maintaining the hermetic sealing. It is over these capacitor sections that most of the high voltage will be dropped.
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| Schematic Diagram of CVT |
A tap voltage (approximately 5-12 kV depending on type) is taken from the lowest capacitor section and fed to an electromagnetic circuit in the cast aluminum base box. The base box contains the intermediate transformer which will provide the final output voltages via multiple tapped secondary windings, series compensating reactor and ferro-resonance control circuitry. The base box is filled with dried mineral oil, protecting the components from environmental deterioration.
Ferro-resonance is simply and effectively controlled by utilization of low flux density designed magnetic circuitry and a saturable reactor controlled damping circuit connected across the secondary winding. The ferro-resonance suppression circuit does not adversely affect transient response.
Main applications of CVTs in HV Networks
The main applications of Capacitor Voltage Transformers in High Voltage Networks ( above 36 kV ) are given below
- Voltage Measuring: They accurately transform transmission voltages down to useable levels for revenue metering, protection and control purposes.
- Insulation: They guarantee the insulation between HV network and LV circuits ensuring safety condition to control room operators.
- HF Transmissions: They can be used for Power Line Carrier (PLC) coupling.
- Transient Recovery Voltage: When installed in close proximity to HV/EHV Circuit Breakers, CVT’s own High Capacitance enhance C/B short line fault / TRV performance.
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